The invention relates to a digital printing or copying machine with at least one fixing device for fixing of toner, which has been transferred to a flexible substrate, where the substrate is bent before fixing.
Digital printing or copying machines are known which utilize for example the electrographic process in which a latent electrostatic image is developed by charged toner particles. The latter are transferred to an image receiving substrate, hereinafter substrate for short. Then the developed image, which has been transferred to the substrate, is fixed by the toner particles being warmed or heated and melted. To melt the toner particles, contact-making methods are often used, in which the toner particles are brought into contact with the corresponding fixing device, for example hot rollers or drums. The disadvantage here is that generally the use of silicone oil as the separating agent, which is designed to prevent adhesion of the melted toner to the heater, is necessary. Furthermore, building and maintaining these contact-making heaters are complex and thus operating costs are high. In addition, the fault rate caused by the contact-making heaters is relatively high. To fix the toner, which has been transferred for example to paper, noncontact heaters and processes are known, in which for example the toner particles are melted using thermal/microwave radiation or hot air.
In the contact making and noncontact melt processes for example toners are used, which have a glass transition temperature TG which is in the range from 45xc2x0 C. to 75xc2x0 C. The glass transition temperature at which the toner, proceeding from the solid state, begins to become soft, can be influenced by the choice of raw materials and by adding certain additives to the toner. In a fixing device, which has at least one heater for the toner, both the toner and also the substrate itself are heated. To be able to ensure good fixing of the toner on the substrate, the surface temperature of the substrate must be in the area of the glass transition temperature of the toner or above. The toner reaches or exceeds the glass transition temperature (TG) in the area of the heater.
Printing and copying machines are known in which the substrate is printed or coated on both sides. For printing of the front and back, the same image generating and transfer device and heater or one separate image generating and transmission device and heater at a time are used. To fix the toner image, the substrate is often guided past at least one image generating and transfer device and the assigned heater using a transport belt on which the substrate lies. First of all, a first toner image is transferred to the first substrate side and fixed. Then a second toner image is transferred to the second substrate side and fixed. When the second toner image is melted, the first substrate side with the already fixed first toner image located on it adjoins the transport belt. The disadvantage here is that while the second toner image is being melted, the first toner image can be heated to such an extent that it becomes soft and tends to stick to the transport belt. This can lead to several undesirable effects: The sticking can lead to damming of the substrate when the substrate is transferred from the transport belt to a following part of the machine. Furthermore, the appearance of the toner image can change in the areas in which it has adhered to the transport belt. This leads to problems in image quality, for example the toner image has a nonuniform gloss.
The object of the invention is to provide a printing machine in which double-sided printing and/or coating of a substrate with simultaneously high quality of the images or coatings applied to the front and back of the substrate is possible.
To achieve this object, a digital printing or copying machine includes at least one fixing device, which has a heater for fixing of a toner image which has been transferred to the substrate. The toner image can be monochrome or polychrome. In conjunction with this invention a xe2x80x9ctoner imagexe2x80x9d is defined also as a coating, which has at least one toner image. The substrate has a certain flexibility and can for example be a sheet or continuous web, for example paper or cardboard. To fix the liquid or dry toner on the substrate, it is routed past the fixing device by a transport, for example a transport belt or a support plate. The printing or copying machine includes a deformation device with which the flexible substrate is bent such that it has at least one arc. The curvature of the substrate is at least so large that its front and back, of which one side can have the toner image to be fixed and the other side can optionally have an already fixed toner image, have no contact with the transporting surface. The construction of the deformation device and its arrangement within the machine are chosen such that the substrate is bent enough at least in sections when it reaches the action area/fixing area of the fixing device. Based on the defined bending of the substrate so that it does not have any mechanical contact with other components or transport devices or the like, it can be ensured that when a toner image located on one substrate side is being fixed, an already fixed toner image located on the other substrate side is not damaged. Therefore uniform image quality and uniform gloss of the toner images on the front and back of the substrate can be ensured.
The deformation device is made such that the substrate is already bent enough before it enters the fixing area or that, as it enters the fixing area, it is continuously bent further and further until it finally has the desired arc. In one advantageous embodiment of the machine, it is provided that the deformation device deforms the substrate in the elastic range. The substrate is therefore not permanently deformed so that it can be smoothed againxe2x80x94preferably automatically as a result of the material-dictated restoration forces of the bent substratexe2x80x94after it has left the action area of the deformation device. Therefore damage to the substrate and the toner image(s) fixed on it can be prevented.
According to one development of the invention, it is provided that the deformation device bends the substrate such that, viewed in the substrate transport direction, it has a convex curvature. The substrate is therefore bent around an imaginary lengthwise center axis which runs parallel to the substrate transport direction, by which the distance of its two lengthwise edges to one another is reduced, while the distance between the front edge and the rear edge of the substrate remains at least essentially the same. In other words, bending of the substrate leads to a reduction of the inside width, viewed in the transport direction.
In one preferred embodiment of the machine, the deformation device has at least two deformation elements which each are assigned to one lengthwise edge or one lengthwise edge area of the substrate and exposes it to transverse forces pointed in the direction of the center of the substrate. The bending of the substrate therefore takes place such that the lengthwise edges of the substrate are moved toward one another. According to one advantageous embodiment, it is provided that the deformation elements are formed by at least one stationary guide rail at a time located along the transport path of the substrate. They are made and aligned to one another such that the substrate transported in the direction of the fixing area of the fixing device with its lengthwise edges or lengthwise edge areas bumps one of the guide rails at a time, with a clearance to one another which becomes smaller in the transport direction. The guide rails therefore have a certain inclination, which can be stipulated relative to the substrate transport direction such that the substrate, which is supported on the guide rails with its lengthwise edges, begins to bend in order to equalize the distance between the guide rails, which is becoming smaller. The distance between the guide rails and their inclination relative to the transport direction of the substrate can preferably be set with a corresponding actuating device. Because the deformation device does not have deformation elements, which can be moved in the transport direction of the substrate, the deformation device can be economically produced.
To impart to the substrate the direction into which it is supposed to bend when its lengthwise edges or its lengthwise edge areas are exposed to forces which are pointed in the direction of the middle of the substrate, in one advantageous embodiment there is at least one nozzle which can be directed against the flat side of the substrate for exposing the substrate to a preferably gaseous jet of a medium. Preferably the substrate is exposed to compressed air.
Furthermore, an embodiment of the machine is preferred, on which, in the free space between the guide rails, there is at least one guide element which is made in a wedge shape, which projects into the substrate transport plane and which is used as a ramp for the substrate. The substrate, which is transported for example by the transport device in the direction of the fixing area, bumps with its front edge against the wedge-shaped guide element which projects into its transport path and slides along its wedge surface. In doing so the substrate is supported with its lengthwise edges/lengthwise edge areas on guide rails, which run toward one another, by which the substrate is bent in the desired manner and direction. The bending direction is therefore dictated by the wedge-shaped guide element.
According to one development of the invention, it is provided that at least one nozzle is integrated into the wedge-shaped guide element to support the bending of the substrate. This yields a space-saving construction.
One embodiment of the machine is also preferred in which the substrate which has at least one arc within the fixing area, especially within the action area of the fixing device heaters for melting the toner, can be moved in the transport direction by the transport device, while maintaining its bent shape. In the preferred embodiment the transport device has several transport elements, which can be moved in the transport direction, of which two at a time are assigned to one lengthwise edge or lengthwise edge area of the substrate, one of the lengthwise edges/lengthwise edge areas at a time being held by clamping between two transport elements at a time. The transport elements can each be formed for example by a transport belt or chain, a transport roller or the like.
In another embodiment of the machine according to this invention, the substrate, which has at least one arc within the fixing area, is guided in the transport direction while maintaining its bent shape by a guide, which has several stationary guide elements. The guide elements are formed for example by guide rails which are aligned parallel to one another and to the substrate transport direction, their distance corresponding to the inside width of the bent substrate. To guide the substrate easily past the fixing device, in one advantageous embodiment it is provided that the substrate transport direction runs in the area of the fixing device in the vertical direction and the substrate is transported by the force of gravity acting on the substrate. The force, which causes displacement of the substrate in the transport direction, is therefore produced among others by the inherent weight of the substrate which has an arc and which is supported on the guide elements. To support the displacement motion of the substrate at least one of its two flat sides can for example also be exposed to compressed air, the compressed air flow having at least one directional component which is pointed perpendicular to the flat side and one component which is parallel to the substrate transport direction.